The invention relates, further, to a once through heat recovery steam generator connected to a steam consumer within a water steam cycle and having an evaporator and at least one superheater section for carrying out the method.
In power stations having a steam generation plant and a steam turbine, the aim in the first place is generally to achieve short start-up times, irrespective of whether a steam power station pure and simple or a combined cycle plant is concerned. What is critical is the duration until steam meeting the requirements of the steam turbine is available under stable conditions at the boiler outlet. Known designs of boilers conform to these requirements only inadequately.
The start-up of the boiler and steam turbine in connection with combined cycle power stations utilizing the exhaust-gas heat of the gas turbine for steam generation in the heat recovery steam generator is particularly important. Gas turbines are nowadays capable of running up from the cold state to the rated-load mode in markedly less than one hour.
Heat recovery steam generators with a circulation-type drum evaporator can often follow such rapid thermal loading to only a restricted extent because of the thick-walled components of the drum. Heat recovery steam generators on the principle of the once-through evaporator, with a separator between the evaporator and superheater, can achieve stable operating states even at medium pressures of up to about 80 bar. However, due to the superheaters connected downstream of the separator, the temperatures at the outlet of the heat recovery steam generator are too high for rapid start-up of a steam turbine.
The time for starting up a steam turbine nevertheless depends essentially on the steam temperatures. The lower the steam temperature can be maintained during start-up, the more rapidly the steam turbine can be started up. The object, for this reason, is to develop heat recovery steam generators, in combination with water steam cycles, which follow the gas turbine without delay during start-up, reach stable operating states quickly and supply the steam turbine at an early stage with steam at low temperatures for the start-up.